MySQL Cannot Add Foreign Key Constraint

• Engine should be the same e.g. InnoDB
• Datatype should be the same, and with same length. e.g. VARCHAR(20)
• Collation Columns charset should be the same. e.g. utf8
  Watchout: Even if your tables have same Collation, columns still could have different one.
• Unique - Foreign key should refer to field that is unique (usually primary key) in the reference table.

I had set one field as "Unsigned" and other one not. Once I set both columns to Unsigned it worked.

To find the specific error run this:

SHOW ENGINE INNODB STATUS;

And look in the LATEST FOREIGN KEY ERROR section.

The data type for the child column must match the parent column exactly. For example, since medicalhistory.MedicalHistoryID is an INT, Patient.MedicalHistory also needs to be an INT, not a SMALLINT.

Also, you should run the query set foreign_key_checks=0 before running the DDL so you can create the tables in an arbitrary order rather than needing to create all parent tables before the relevant child tables.